An ultrasonic diagnostic apparatus transmits an ultrasonic beam from a ultrasonic probe to a subject, and obtains information necessary for diagnosis on the basis of a reception signal from the subject. For example, a Doppler frequency shift in a reception signal is detected and, on the basis of the Doppler frequency shift, the velocity of blood flow can be obtained.
Such an ultrasonic Doppler apparatus has, as shown in FIG. 9, a probe 1 for transmitting/receiving ultrasonic wave, an interface 5 for operating an ultrasonic diagnostic apparatus, a control unit 4 for controlling the whole system, a pulse/CW transmit signal generator 3 for generating a transmission waveform on the basis of the control unit 4, a transmitter 2 for amplifying a signal generated by the pulse/CW transmit signal generator 3, a pre-amplifier 6 for amplifying a reception signal from the probe, a receive beamformer 7 for selectively emphasizing a signal from a desired location, a Doppler processing unit 8 for detecting a Doppler shift component from the blood flow from a phase output, a digital scan converter (DSC) 10 for displaying an instantaneous frequency component calculated by the processing unit as blood flow waveform data, and a display 11 for displaying an output of the scan converter.
In such an ultrasonic Doppler apparatus, when ultrasonic wave having a frequency f0 is output from the probe 1, since blood of the subject flows at predetermined speed, an echo signal obtained by the ultrasonic wave from the probe 1 reflected by blood cells has been subjected to a frequency shift by the Doppler effect. The Doppler processing unit 8 detects the Doppler shift component (Doppler signal). Since the blood flow velocity varies, a Doppler signal obtained includes different frequency components. The Doppler processing unit 8 obtains a frequency distribution, that is, blood flow distribution by performing a method such as fast Fourier transform on the Doppler shift component. By sequentially performing the computation, a change with time in the blood flow distribution is displayed. The method provides significant data also from the clinical viewpoint and is widely used. In particular, the data can be used not only as quantitative data of the maximum blood flow velocity in a predetermined position in a subject but also as data in the case of diagnosing a valvular disease of the heart or the like. By displaying a time-varying waveform of a velocity distribution of the blood flow from the portion of the valve of the heat, backflow at the valve can be determined. The shape itself of the time-varying waveform of the blood flow velocity distribution is widely known as data useful for diagnosis.
Since a Doppler signal is based on a reflection signal from a blood cell having low ultrasonic scattering power, there has been the task of improving a signal-to-noise ratio. Noise exists also in a velocity range in which signals do not inherently exit in the time-varying waveform of the blood flow velocity distribution, and it largely deteriorates visibility of the time-varying waveform of the blood flow velocity distribution. As a method of solving the problem, for example, a method of setting a threshold between an area including many noise components and an area including many signal components at the time of performing gray scale mapping has been proposed (refer to, for example, Published Japanese Translation of PCT International Publication for Patent Application No. 2002-534185). The gray scale mapping is a method of displaying a change with time of a blood flow velocity distribution by setting time on the axis of abscissa, velocity on the axis of ordinate, and intensity of a signal as brightness.